Offshore searching for the deformation front extended from onland Tainan, Taiwan

Taiwan is at the boundary between the Eurasian Plate and the Philippine Sea Plate, and the Philippine Sea Plate is moving northwest toward the Eurasia Plate at a rate of ~72 mm/yr. The collision between them resulting in Taiwan orogeny, and forming a fold-and-thrust belt aglined in nouth-south direction. Therefore, the western plains and foothills of Taiwan have formed by the action of the north-south strike reverse faults in the belt. More to the west, the age of fault is younger , and the dip is smaller. These structure lines will extend south to Kaoping area. Most geologists believe that the youngest Changhua fault extends southly to Tainan area, but becomes fragmented further to south; therefore, fault line is hard to located especially in Tainan area. According to the reflection seismic profiles, in the continental slope offshore the Tainan and Kaohsiung areas, some researchers believe the paleo-trace of the Manila Trench can be extending northwards and connected to the Changhua fault, marking a deformation front going from land to ocean floor. However, their conclusions lack nearshore topographical and geophysical data such that the doublt has been prevailed to support the Manila Trench was previously located in Taiwan Orogon. Therefore, nearshore geophysical survey was conducted off Tainan coast, firstly, the R2 Multi-beam Sounder was used and the significant topographical lineations related with those geological structures shown onland Tainan are found. Besides, by using 3.5kHz bottom profiler, the dislocation in shallow sediments is also dominant. This study indicates there is less evidence to suggest the deformation front formed along Taiwan Isand can extends offshore into the continental slope and shall connect with the Manila Trench.

Key words: deformation front, fold-and-thrust belt, Manila trench , geophysical prospecting